A Study of the Leptospiral Carrier State of Rats

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Abstract

The problem. The Common Norway rat ("Rattus norvegicus") is a known carrier and shedder of "Leptospira" of the serotype "icterohaerrorrhagiae". This investigation was done to determine if the Des Moines wild rat population was infected with "L. icterohaemorrhagiae". The virulence of any isolate was to be tested in laboratory hamsters and rats.
Procedure. Wild Des Moines rats were captured and euthanized, weighed, sexed, and measured for total length. Various tissues were cultured for isolation of infecting leptospires. Leptospiral antibodies were determined from serum samples. Histological examination of rat kidney tissue was done. The LD5O value of the wild isolate was determined in laboratory hamsters. Laboratory rats were challenged with a high and low dose of the wild isolate, and then monitored for a serologic
and histologic response.
Findings. From the 14 infected wild rats, positive leptospiral isolations were made in 100% of the kidney tissues and 57% of the urine.
Eleven of the 14 infected wild rats had antibody titers against "L. icterohaemorrhagiae" ranging from 1:80 to 1:1000. Renal involvement was
observed histologically in infected wild rats. The LD50 titer in hamsters was determined to be 10 5.3/ml. Laboratory rats challenged with a
high concentration of the wild isolate responded with a titer of 1:80.
Conclusion. Eighteen percent of the wild rat population sampled was infected with leptospires. The microscopic agglutination test determined the isolate to be a member of the "icterohaerrorrhagiae" serotype. Immature wild rats (under 200 grams) were not infected with leptospires. The wild isolate was found to be lethal to weanling hamsters.
Recommendations. Larger samples of the Des Moines wild rat population are needed in order to determine whether the rats infected with leptospires are localized or dispersed throughout the city. A larger sample may also show evidence of leptospiral serotypes other than "icterohaerrorrhagiae". The immunity from "L. icterohaemorrhagiae" in young wild and laboratory rats should be studied further to determine the
cause of immunity.